A number of devices of this type are already known in the art, such as, for example, superconducting quantum interferometers (Superconducting Quantum Interferences Devices or "SQUIDs"). The operation of these interferometers is based on phenomena linked to the superconductivity of the materials used, such as magnetic flux quantization and the Josephson effects.
In fact, when a ring of metallic material is cooled until it passes into the superconducting state, in the presence of a magnetic field, and then this field is cut, it has been noted that the magnetic flux, which quantity represents the product of the field in the ring and the area of the latter, remains trapped in the ring. The flux is held by a superconduction current which has been created in the ring to oppose the flux variations, according to Lenz's law, and persists there permanently. Thus, the current induced in the ring is representative of the flux and therefore of the magnetic field which it is desired to determine. Measuring this current therefore makes it possible to deduce the magnetic field. However, this measurement has some drawbacks.
In fact, in a so-called direct-current Josephson-effect interferometer, the superconducting ring includes two barriers. When a magnetic field is applied, these become resistive and a potential difference is created between the parts of the ring separated by the barriers. The potential difference can then be measured and makes it possible to determine the magnetic flux.
In so-called alternating-current Josephson-effect interferometers, the ring has only a single barrier and is therefore easier to manufacture. However, it is more difficult to detect the entry of the barrier into the resistive state, and this problem is solved by exploiting the low inductance of the ring. This ring is in fact coupled by mutual induction to an external oscillating circuit which induces an alternating current therein at frequencies ranging from ten MHz up to ultra-high frequencies. The variations of the current in the ring, which are induced by the magnetic field to be measured, are then manifested by variations in the level of oscillation of the external circuit, which variations can be measured in order to determine the magnetic field.
It is conceivable that this type of superconducting quantum interferometer using the Josephson effect makes it possible to detect only variations in magnetic flux. These high-performance devices require, for their production, control of techniques which are highly advanced and therefore very expensive. They are actually extremely difficult to manufacture insofar as it is necessary to produce tunnel junctions constituting the barriers described above and to use highly complex associated measurement electronics.
For further teaching on these interferometers, reference should be made to the articles published in the journals "La Recherche", No. 133 (May 1982), pages 604 to 613, and "Pour la Science", No. 138 (April 1989), pages 52 to 61.
A cryogenic magnetometer is also known, for example from U.S. Pat. No. 3,437,919. This magnetometer comprises a detection coil arranged between two discs of superconducting material. The discs are held at a temperature of less than 10.degree. K. in a cryostat filled with liquid helium.
The temperature of the discs is periodically and simultaneously altered so that it is alternately less than and greater than their superconducting transition temperature.
Because of this, the magnetic flux to be measured passes through the discs and the detection coil at the same frequency as the temperature changes of the discs.
A magnetometer is also known, from FR-A 2,648,568, in which a thin film of superconducting material, which is for example in the shape of a disc, is heated using light radiation transmitted by an optical fiber. The thin film of superconducting material then passes alternately above and below its superconducting transition temperature, which allows the flux lines of the magnetic field to be measured to pass. A measurement coil is arranged around the thin film in order to detect the magnetic field variations.
However, the application of these devices to the detection and measurement of very weak magnetic fields requires complete thermal transition cycles at high frequency over the entire surface of the discs. A very great difficulty then arises, this difficulty consisting in not trapping the field lines in a disc when it is cooled below its superconducting transition temperature.
To avoid this trapping, it is necessary for the cooling to occur evenly from the center of the disc to its periphery.
However, controlling this regulation limits the thermal modulation frequency and therefore the sensitivity of the device.
The object of the invention is to solve these problems.